Experts from the Asia Maritime Transparency Initiative (AMTI) said a video from the Chinese Communist Party's People's Daily newspaper showed an H-6K landing and taking off from a base on Woody Island, the largest of the Paracel Islands.

Woody Island, which China calls Yongxing, is also claimed by Vietnam and Taiwan.

Although China has deployed fighter jets to Woody Island in the past, this is the first time bombers have landed on a South China Sea island, the AMTI said. It added that an H-6K could reach all of South East Asia from the island.

Analysts say bombers will likely soon land in the Spratly Islands further south, where runways and hangars have been built on reefs.

From there H-6Ks could reach northern Australia or US bases on Guam, says the AMTI.

The US has sailed warships close to artificial islands built by Beijing in the South China Sea to challenge what it sees as Chinese efforts to restrict freedom of navigation in a strategically important area.

A Pentagon spokesman told Reuters news agency that the US "remains committed to a free and open Indo-Pacific".

"We have seen these same reports and China's continued militarisation of disputed features in the South China Sea only serves to raise tensions and destabilise the region," Lt Col Christopher Logan said.

The South China Sea dispute

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Media captionRupert Wingfield-Hayes says China is determined to assert its control

Sovereignty over two largely uninhabited island chains, the Paracels and the Spratlys, is disputed by China, Vietnam, the Philippines, Taiwan and Malaysia

China claims the largest portion of territory, saying its rights go back centuries - in 1947 it issued a map detailing its claims

The area is a major shipping route, and a rich fishing ground, and is thought to have abundant oil and gas reserves